During the past several years, Canada has been developing a minefield breaching system based on the concept of fuel-air explosives (FAE). The system has been named "Fuel-Air Line-Charge Ordnance Neutralizer", or FALCON, for which Canadian, United States and European patent applications have been filed. The phenomenon of FAE is a very attractive option for weapons in that a fuel-air cloud covers a large area and produces a strong blast wave. Once detonated, one kilogram of dispersed fuel can generate a blast wave equivalent to that produced by more than five kilograms of TNT.
A conventional FAE event consists of two stages. In the first stage, the fuel is explosively disseminated to form a large fuel-air cloud. Subsequently, in stage two, a high-explosive secondary charge is detonated to generate a shock wave which, in turn, initiates detonation of the dispersed medium.
Examples of the conventional FAE system are found in the above-referenced FALCON patent applications (e.g., U.S. application Ser. No. 389,747 filed on Aug. 4, 1989, now U.S. Pat. No. 4,967,636) and in U.S. Pat. No. 3,724,319; French Patents 2,014,848 and 2,226,064; British 2,199,289 A; Swiss 387,494; and E.P.O. published application 0,232,194.
Typically, a conventional minefield breaching system involves the provision of elongated fuel-carrying means, such as a flexible hose or a plurality of interconnected canisters, that can be laid on a minefield without disturbing the mines. A small rocket, for example, can tow the fuel-carrying means across the minefield with the fuel-carrying means descending by parachute as the rocket comes to earth. Thereafter the fuel is dispersed by the burster charge to create the cloud of fuel droplets-in-air (stage 1) and then a secondary charge is detonated to effect detonation of the dispersed cloud (stage 2). The extremely high pressures created upon cloud detonation will neutralize the mines along the path of the cloud, either by causing them to explode or by rendering them useless, so that men and materiel can cross the minefield along the cleared path.
In the interests of increasing the reliability of FAE devices, while at the same time reducing their size, weight, cost and engineering complexity, a significant effort has been directed toward the development of a "Single-Event" FAE device; that is, one which disperses the fuel into a large cloud that detonates automatically after a prescribed delay time. There is much incentive to eliminate the secondary charges from FAE munitions because these charges are often ejected into the developing fuel-air cloud as the munition approaches the target at high speed. Many weapon system failures have been attributed to the charges being ejected outside the cloud, or detonating in regions of overly rich or lean fuel-air mixture.
If the high-explosive secondary charges, which constitute a strong initiation source, are eliminated from a FAE device, then one must rely on weak ignition (e.g., a mild flame) followed by some method of amplifying a weak compression wave to a shock wave of detonation proportions. Although this phenomenon has been observed experimentally, it is not well understood. In conventional blast initiation of detonation, free radicals for the oxidation processes are brought about by thermal dissociation in the wake of a strong shock wave generated by a powerful energy source. Successful initiation depends on both the shock strength and duration, with the minimum values of these parameters depending on the sensitivity of the combustible mixture. If the initiation source is too weak, chemical reactions can still take place. However, auto-ignition of the mixture may occur too late for the liberated energy to be of use in supporting the leading shock. If detonation is to occur under such circumstances, some means of shock wave amplification, leading to transition from deflagration to detonation (DDT), must come into play.
An important clue in identifying the critical conditions for the onset of detonation can be drawn from observations about initiation in the wake of a reflected shock wave from the end wall of a tube. In this scenario, the fluid particles are heated initially by the incident wave and heated further by the reflected wave. After an induction time, the particles ignite. Although the induction time is the same for all particles in the wake of the reflected wave, ignition occurs in a definite time sequence. The lamina of gas immediately adjacent to the end wall, having been processed first, will be the first to explode. The resulting weak shock wave propagates into the neighboring lamina which, having been processed slightly later in time, will itself be on the verge of exploding. The resulting higher-strength shock wave generated by this second explosion propagates into yet a third lamina where the process is repeated. Although it is not clear whether the shock entering a given lamina actually triggers the explosion or simply arrives there at the precise moment the explosion takes place, it is nonetheless this continuous time sequence of energy release that provides the mechanism for shock wave amplification. In order for amplification to occur, the sequence must be such that the chemical energy release at time t makes an effective contribution to the shock wave produced by the energy release at times less than t. Thus, the phenomenon is one of "shock wave amplification by coherent energy release", or SWACER (Lee et al., 1978). This concept suggests that, given a certain amount of available chemical energy, the optimal means of generating a strong shock wave is not to release it instantaneously and uniformly over a region.
Various means of arranging the appropriate temporal and spatial energy release sequence have been examined. Zeldovich and colleagues (1970) carried out a numerical study of detonation in non-uniformly preheated gas mixtures. For the case of a mild temperature gradient, the pressure rise in the test volume was uniform and substantially less than the detonation pressure. In the other extreme of a steep temperature gradient, the shock wave and reaction zone were seen to decouple, leading to a deflagration. Between these two limits, there existed a range of gradients for which the onset of detonation was observed.
The SWACER concept was first proposed as the mechanism responsible for the photo-chemical initiation of H.sub.2 --Cl.sub.2 mixtures (Lee et al., 1978; Yoshikawa, 1980). In this study, the energy release sequence was determined by the gradient in chlorine atom concentration produced by the photo-dissociation of Cl.sub.2 by a flashlamp. Owing to the absorption of light by the gas, the Cl concentration decreased in the direction of the light beam, resulting in a sequence of energy release determined by the dependence of induction time on the Cl concentration. For low flashlamp intensities (steep Cl concentration gradients), no detonation was formed while, for very high intensities (leading to uniform irradiation of the volume), the process approached that of constant volume combustion. Between these two extremes, a range of intensities was identified for which detonation was possible.
The experimental observation that rapid turbulent mixing between combustion products and unburned explosive mixture can lead to detonation provides further support for the SWACER mechanism. In a study by Knystautas et al. (1979), such mixing within large turbulent eddies led to both a temperature gradient and a free-radical concentration gradient. For a large enough eddy and an appropriate turbulent mixing time with respect to the chemical kinetic time scales, detonation was seen to result. The same mechanism was likely operative in the recent investigations by Moen et al (1988), Mackay et al. (1988), and Ungut and Shuff (1989). These authors reported transition to detonation near the exit of a tube following entrainment of hot combustion products into the starting ring vortex ahead of the flame.
Experiments carried out by Lee and co-workers (1979) have shown that the conditions for the onset of detonation can also be realized in the turbulent mixing region generated by opposing reactive gas jets; one containing propane and the other containing a fluorine-oxygen mixture. In these experiments, the delay to ignition was observed to depend on the amount of fluorine present. The chemistry of both the F.sub.2 --C.sub.3 H.sub.8 --O.sub.2 and F.sub.2 --C.sub.4 H.sub.10 --O.sub.2 systems has been studied in detail by von Elbe (1974). The study reported by Urtiew et al. (1977) was similar except that the time to the onset of detonation was controlled by the use of an inhibitor, rather than a sensitizer. Tetrafluorohydrazine and silane, which normally react in a nearly instantaneous fashion, were employed in these experiments. However, by using a cis-2-butene inhibitor, the reaction was delayed to allow turbulent mixing within a volume exceeding the critical detonable volume for the mixture. Ignition was seen to occur in a localized region of inhibitor deficiency, followed by shock wave amplification through the region of induction-time gradient.
All of the above-mentioned studies which have led to initiation of detonation by induced chemical sensitization have involved relatively sensitive fuel-oxidizer systems. Although attempts have been made to initiate less sensitive fuel-air mixtures (e.g., Tulis, 1978; von Elbe and McHale, 1979; Sayles, 1984), there is little evidence to suggest that self-sustained detonation has actually been achieved, albeit significant overpressures have been measured.